1. Field of the Invention
The present invention relates to a filter adjustment circuit that adjusts the quality factor (Q factor) of an active filter, which incorporates an amplifier.
2. Background Art
Recently, active filters have been required to have a wider band. However, it is difficult to manufacture active filters having a wide band and a high Q factor as designed without variations in Q factor.
The Q factor varies due to variations of passive elements, such as resistors “R” capacitors “C” and inductors “L”. In addition, the Q factor of the active filter varies due to a nonideal effect of the amplifier. While an ideal amplifier has infinite gain and band, an actual amplifier has finite gain and band.
For example, a conventional filter adjustment circuit comprises an input terminal, a reference signal generator, a filter (gm-C filter) having a control terminal for outputting a control signal for controlling filter characteristics, an output terminal for outputting a signal that has been input at the input terminal and passed through the filter, an amplitude detector section that receives the output signal of the filter and detects the amplitude of the signal, a phase comparator section that detects the phase difference between the signal before passing through the filter and the signal after passing through the filter, and a control section that inputs a reference signal to the filter, successively inputs a plurality of control signals to the control terminal of the filter, and determines the value of the control signal for normal use based on each amplitude detected by the amplitude detector section and each phase difference detected by the phase comparator section.
This filter adjustment circuit configured as described above adjusts the characteristic frequency and the Q factor of the gm-C filter (see “An Accurate Quality Factor Tuning Scheme for IF and High-Q Continuous-Time Filters”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 33, NO. 12, DECEMBER 1998, for example).
However, the prior-art literature discloses only a method of adjusting the characteristic frequency of a gm-C filter and does not discloses any specific circuit configuration for adjusting the Q factor of an active filter.
Therefore, according to the prior art, when the Q factor varies due to the manufacturing condition or the like or due to a parasitic effect or a nonideal effect of the amplifier, the Q factor cannot be adjusted automatically.
As described above, conventional filter adjustment circuits have a disadvantage that it is difficult to produce a desired Q factor of a filter.